The painted turtle becomes sexually mature when it reaches a certain size, not age. The female has to carry the egg, so must be at least 5 inches before she can reproduce for the first time. This typically occurs between 4 and 8 years after birth (Shiels 2000). The males does not need to be so large, so they normally becomes sexually mature at a younger age, ranging from 2 to 4 years (Dawson 2000). Studies have found that the temperature during growing season or the length of the growing season can effect the rate of maturity in painted turtles. When growing seasons are longer or have warmer average temperatures, the painted turtles grow faster and reach sexual maturity at younger ages (Frazer 1993).
Depending on how far north the turtles live, the courting and nesting can occur anywhere from May to July. In the south, it takes place earlier and females may lay up to 4 hatches each year. In the north, it happens later when the temperature warms up, meaning there is only time for females to lay one hatch (Dawson 2000). A typical hatch consists of 5 to 10 eggs, although larger sub-species can lay up to 20 eggs per hatch, and smaller sub-species can have hatches of only 2 eggs (Dawson 2000, Kohen 1992).
The female lays the eggs in 8-10 cm flask-shaped nests that she digs with her back feet. The nests can be dug in various soil types, which can affect the survival of the hatchling (see New Hypothesis page). Normally, they are on sloping banks with southern exposure since this helps the incubation (Storey and Storey 1992). The incubation period lasts an average of 76 days. Evidence shows that painted turtles exhibit temperature-dependent sex determination, meaning incubation temperature effects the sex of the hatchlings (Kohen 1992, Janzen and Morjan 2001). At 30.5*C, the eggs produce female hatchlings. At 25*C, the eggs produce male hatchlings, and at 29*C both females and males hatch (Kohen 1992).
When the hatchlings emerge from their eggs, they are around 1 inch long, and their markings are brighter than those of mature turtles (Kohen 1992).
Painted turtle hatchlings. Photographs by J.D. Wilson, used with permission of Michael Dorcas.
As opposed to other turtle hatchlings, many painted turtles overwinter in their nest. The reason the hatchlings chose to remain in the nest is still under research. It is widely accepted that the painted turtle remained in the nest because there is more food available and less predation in the spring. However, it has recently been found that the real reason may have more to do with evolutionary constraints. They found that painted turtle hatchlings forced to overwinter in water (as adult painted turtles do: see After the First Winter page) had a much lower survival rate than those that overwintered in soil, perhaps due to an inability to absorb oxygen like the adults can (Packard et. al. 2001). Whichever the reason for remaining in the nest, doing so exposes hatchlings (especially those in the north) to temperatures below their body fluid freezing point of -0.7 degrees Celsius. The strategy that these turtles use to survive these temperatures has been a topic of research for many years.